A bisimide compound is a raw material which has been and is used in various fields as a precursor for a thermosetting, adduct-type polyimide resin. This compound overcomes the incompleteness of imidation which is a problem in using a linear polyimide resin obtained via a conventional polyamic acid. When the imidation is incomplete, that is, when the inclosure ratio of imide group is low, the glass transition point (Tg) decreases to decrease the heat resistance, and the resistance to hydrolysis decreases. This bisimide compound is also a raw material which has processability almost equivalent to that of an epoxy resin and is widely used in a hot melt, a prepreg and a vulcanizer for rubbers.
A maleimide resin obtained from a bisimide compound typified by N,N'-(methylene-di-p-phenylene)bismaleimide is known as a resin excellent in heat resistance. For example, a polymaleimide resin which is a homopolymer of bismaleimide, or a polymaleimide-polyamine resin which is obtained by polymerization with an amine is widely used in an impregnating varnish, a laminated product and a molding material.
A bisimide compound often has a structure containing an aromatic group which is a rigid structure. There has been therefore a problem in that it is poor in flexibility after being cured and that it is extremely poor in solubility in a solvent. Further, there is also a problem in that it is difficult to adjust the glass transition point thereof. For overcoming the poor solubility in a solvent, attempts have been so far made in various ways, such as an attempt to use a Michel addition reaction between a bisimide compound and an aliphatic diamine or an aromatic diamine or an attempt to design the structure of a diamine as a raw material used for the synthesis of a bisimide.
However, as compared with studies on the solubility, almost no effort has been made to decrease the melting point of a bisimide compound which has not yet cured. If there is provided a bisimide compound having excellent solubility in a solvent and having a low melting point, there is an advantage that the adhesion temperature of the bisimide compound can be decreased, etc.
For decreasing the reel ting point of a bisimide compound, there is a method in which a structure rich with flexibility such as an alkylene structure is introduced into the main skeleton in place of an aromatic ring which is a rigid structure. However, it is very difficult to use aliphatic diamines as raw materials for the bisimide compound, since the aliphatic diamines have high reactivity. That is, it is known that the aliphatic diamines undergo undesirable side reactions to have high molecular weights. For this reason, for the general synthesis of a bisimide compound, an aromatic diamine is generally used as a raw material. That is, a generally synthesized bisimide compound has a main skeleton containing the structure of an aromatic diamine. For decreasing the melting point of this bisimide compound, it has been necessary to introduce a substituent alkyl group into the aromatic ring, or to introduce a long-chain alkylene group as a binding group between aromatic rings. In these cases, however, the solubility in a solvent has been insufficient.
It is an object of the present invention to provide a novel bisimide compound having a decreased melting point, and a process for the production thereof. It is a further object of the present invention to provide a novel bisimide compound which does not impair high-temperature stability such as temperature for heat decomposition initiation, has excellent solubility in a solvent and has a decreased melting point.